In general, lithography refers to processes for pattern transfer between various media. A lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Typically, a "transparency" of the subject pattern is made having areas which are selectively transparent, opaque, reflective, or non-reflective to the "projecting" radiation. Exposure of the coating through the transparency causes the image area to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (i.e., uncrosslinked) areas are removed in the developing process to leave the pattern image in the coating as less soluble crosslinked polymer.
Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits caused by the wavelengths of the optical radiation. "Long" or "soft" x-rays (a.k.a. Extreme UV) (wavelength range of .lambda.=100 to 200 .ANG. ("Angstrom") are now at the forefront of research in efforts to achieve the smaller desired feature sizes. Soft x-ray radiation, however, has its own problems. The complicated and precise optical lens systems used in conventional projection lithography do not work well for a variety of reasons. Chief among them is the fact that there are no transparent, non-absorbing lens materials for soft x-rays and most x-ray reflectors have efficiencies of only about 70%, which in itself dictates very simple beam guiding optics with very few surfaces.
One approach has been to develop cameras that use only a few surfaces and can image with acuity (i.e., sharpness of sense perception) only along a narrow arc or ringfield. Such cameras then scan a reflective mask across the ringfield and translate the image onto a scanned wafer for processing. Although cameras have been designed for ringfield scanning (e. g., Jewell et al., U.S. Pat. No. 5,3 15,629 and Offner, U.S. Pat. No. 3,748,015), available condensers that can efficiently couple the light from a synchrotron source to the ringfield required by this type of camera have not been fully explored. Furthermore, full field imaging, as opposed to ringfield imaging, requires severely aspheric mirrors in the camera. Such mirrors cannot be manufactured to the necessary tolerances with present technology for use at the required wavelengths.
The present state-of-the-art for Very Large Scale Integration ("VLSI") involves chips with circuitry built to design rules of 0.25 .mu.m. Effort directed to further miniaturization takes the initial form of more fully utilizing the resolution capability of presently-used ultraviolet ("UV") delineating radiation. "Deep UV" (wavelength range of .lambda.=0.3 .mu.m to 0.1 .mu.m), with techniques such as phase masking, off-axis illumination, and step-and-repeat may permit design rules (minimum feature or space dimension) of 0.18 .mu.m or slightly smaller.
To achieve still smaller design rules, a different form of delineating radiation is required to avoid wavelength-related resolution limits. One research path is to utilize electron or other charged-particle radiation. Use of electromagnetic radiation for this purpose will require x-ray wavelengths.
Two x-ray radiation sources are under consideration. One source, a plasma x-ray source, depends upon a high power, pulsed laser (e.g., a yttrium aluminum garnet ("YAG") laser), or an excimer laser, delivering 500 to 1,000 watts of power to a 50 .mu.m to 250 .mu.m spot, thereby heating a source material to, for example, 250,000.degree. C., to emit x-ray radiation from the resulting plasma. Plasma sources are compact, and may be dedicated to a single production line (so that malfunction does not close down the entire plant). Another source, the electron storage ring synchrotron, has been used for many years and is at an advanced stage of development. Synchrotrons are particularly promising sources of x-rays for lithography because they provide very stable and defined sources of x-rays.
Electrons, accelerated to relativistic velocity, follow their magnetic-field-constrained orbit inside a vacuum enclosure of the synchrotron and emit electromagnetic radiation as they are bent by a magnetic field used to define their path of travel. Radiation, in the wavelength range of consequence for lithography, is reliably produced. The synchrotron produces precisely defined radiation to meet the demands of extremely sophisticated experimentation. The electromagnetic radiation emitted by the electrons is an unavoidable consequence of changing the direction of travel of the electrons and is typically referred to as synchrotron radiation. Synchrotron radiation is comprised of electromagnetic waves of very strong directivity emitted when electron or positron particles, traveling within the synchrotron at near light velocity, are deflected from their orbits by a magnetic field.
Synchrotron radiation is emitted in a continuous spectrum or fan of "light", referred to as synchrotron emission light, ranging from radio and infrared wavelengths upwards through the spectrum, without the intense, narrow peaks associated with other sources. Synchrotron emission light has characteristics such that the beam intensity is high, and the divergence is small so that it becomes possible to accurately and deeply sensitize a photolithographic mask pattern into a thickly applied resist. Generally, all synchrotrons have spectral curves similar to the shape shown in FIG. 1 of Cerrina et al. (U.S. Pat. No. 5,371,774) that define their spectra. The particular intensity and critical photon energy will vary among different synchrotrons depending upon the machine parameters.
Parameters describing the size of the source of synchrotron radiation and the rate at which it is diverging from the source are of importance. Because the electrons are the source of synchrotron radiation, the cross section of the electron beam defines the cross section of the source. Within the plane of the orbit, the light is emitted in a broad, continuous fan, which is tangent to the path of the electrons.
Because of the relatively small height and width of the electron beam, any point along its length acts as a point source of radiation, providing crisp images at an exposure plane which is typically 8 meters or more away from the ring. At a distance of 8 meters, however, a 1 inch wide exposure field typically collects only 3.2 milli-radians ("mrad") of the available radiation. There are two ways to improve the power incident at a photo-resist: either shorten the beamline or install focusing elements. The use of focusing elements has the potential advantage of collecting x-rays from a very wide aperture and providing a wide image with a very small vertical height. However, the use of focusing elements results in a loss of power at each element because of low reflectivity of the x-rays and introduces aberrations. Synchrotron radiation is emitted in a horizontal fan. The small vertical divergence of the synchrotron radiation implies that a wide horizontal mirror, or a plurality of smaller parallel systems, can accept a large fan of light, whose outputs are added together at the mask plane.
A variety of x-ray patterning approaches are under study. Probably the most developed form of x-ray lithography is proximity printing. In proximity printing, object:image size ratio is necessarily limited to a 1:1 ratio and is produced much in the manner of photographic contact printing. A fine-membrane mask is maintained at one or a few microns spacing from the wafer (i.e., out of contact with the wafer, thus, the term "proximity"), which lessens the likelihood of mask damage but does not eliminate it. Making perfect masks on a fragile membrane continues to be a major problem. Necessary absence of optics in-between the mask and the wafer necessitates a high level of parallelism (or collimation) in the incident radiation. X-ray radiation of wavelength .lambda..ltoreq.16 .ANG. is required for 0.25 .mu.m or smaller patterning to limit diffraction at feature edges on the mask.
Use has been made of the synchrotron source in proximity printing. Consistent with traditional, highly demanding, scientific usage, proximity printing has been based on the usual small collection arc. Relatively small power resulting from the 10 mrad to 20 mrad arc of collection, together with the high-aspect ratio of the synchrotron emission light, has led to use of a scanning high-aspect ratio illumination field (rather than the use of a full-field imaging field).
Projection lithography has natural advantages over proximity printing. One advantage is that the likelihood of mask damage is reduced, which reduces the cost of the now larger-feature mask. Imaging or camera optics in-between the mask and the wafer compensate for edge scattering and, so, permit use of longer wavelength radiation. Use of extreme ultra-violet radiation (a.k.a., soft x-rays) increases the permitted angle of incidence for glancing-angle optics. The resulting system is known as extreme UV ("EUVL") lithography (a.k.a., soft x-ray projection lithography ("SXPL")).
A favored form of EUVL is ringfield scanning. All ringfield optical forms are based on radial dependence of aberration and use the technique of balancing low order aberrations, i.e., third order aberrations, with higher order aberrations to create long, narrow illumination fields or annular regions of correction away from the optical axis of the system (regions of constant radius, rotationally symmetric with respect to the axis). Consequently, the shape of the corrected region is an arcuate or curved strip rather than a straight strip. The arcuate strip is a segment of the circular ring with its center of revolution at the optic axis of the camera. See FIG. 4 of U.S. Pat. No. 5,315,629 for an exemplary schematic representation of an arcuate slit defined by width, W, and length, L, and depicted as a portion of a ringfield defined by radial dimension, R, spanning the distance from an optic axis and the center of the arcuate slit. The strip width is a function of the smallest feature to be printed with increasing residual astigmatism, distortion, and Petzval curvature at distances greater or smaller than the design radius being of greater consequence for greater resolution. Use of such an arcuate field allows minimization of radially-dependent image aberrations in the image. Use of object:image size reduction of, for example, 5:1 reduction, results in significant cost reduction of the, now, enlarged-feature mask.
It is expected that effort toward adaptation of electron storage ring. synchrotron sources for EUVL will continue. Economical high-throughput fabrication of 0.25 .mu.m or smaller design-rule devices is made possible by use of synchrotron-derived x-ray delineating radiation. Large angle collection over at least 100 mrad will be important for device fabrication. Design of collection and processing optics for the condenser is complicated by the severe mismatch between the synchrotron light emission pattern and that of the ringfield scan line.
Sweatt, U.S. Pat. No. 5,512,759, discloses a condenser for collecting and processing illumination from a synchrotron source and directing the illumination into a ringfield camera designed for photolithography. The condenser employs a relatively simple and inexpensive design, which utilizes spherical and flat mirrors that are easily manufactured. The condenser employs a plurality of optical mirrors and lenses, which form collecting, processing, and imaging optics to accomplish this objective.
Sweatt, U.S. Pat. No. 5,361,292, discloses a condenser that includes a series of aspheric mirrors on one side of a small, incoherent source of radiation. If the mirrors were continuously joined into a parent mirror, they would image the quasi point source into a ring image with a diameter of a few tens of centimeters at some distance, here some number of meters. Since only a relatively small arc (about 60 degrees) of the ring image is needed by the camera, the most efficient solution is to have about five 60.degree. beams, all of which are manipulated such that they all fall onto the same arc needed by the camera. Also, all of the beams must be aimed through the camera's virtual entrance pupil. These requirements are met in two steps.
First, the beams are individually rotated and translated, as necessary, using mirrors so that they overlap at the ringfield and pass through the real entrance pupil without interfering with each other. The second step is to image this real entrance pupil into the camera's virtual entrance pupil using a powered imaging mirror. This places the corrected, combined images of the mirrors into the proper position for use by the camera. This system may be configured in a variety of ways.
Despite the advantages in condenser designs for projection lithography, the art is in search of techniques that enhance critical dimension (CD) control.
Condenser designs such as those disclosed by Sweatt are quite efficient in that they collect about 1 steradian of light from a small point incoherent source or 20.degree. or 30.degree. from a synchrotron and pass it through the ringfield and into the entrance pupil. However, the illumination pattern in the entrance pupil is far from uniform causing a large CD error.
The 3-nm CD error typically obtained with 100-nm features using current techniques is quite large and requires that the masks be designed and then modified to remove these predictable errors. This mask modification step is an iterative procedure where the mask geometry corrections are estimated and then the corrected mask must be modeled. This step adds expense and uncertainty to the function of the completed chips, even possibly requiring a redesign and the fabrication of a new set of masks.
In particular, condenser improvements are sought that would permit mask designers to lay out the features on the mask essentially exactly as they are to be on the wafer. This will reduce the mask layout costs and make extreme-UV lithography more attractive.